React fics thrive on the interior voice of the viewpoint character, I think. When a character is reacting, you're inside their head watching them parse new information, and if their thoughts are just a flat 'Wow, cool,' it dies. The trick is layering their interpretation through their established personality. A pragmatic character might immediately inventory tactical weaknesses in a revealed magic system, while a romantic one might zero in on a subtle glance between two other characters everyone else missed.
Dialogue from the non-reacting characters needs to land with weight, too. It's not enough to have someone drop a bombshell line; the reacting character's internal monologue should briefly replay it, maybe even mishear it in shock, or latch onto a single word that changes everything for them. I've seen great scenes where the reaction is mostly silence, but the description of the physical sensations—ears ringing, stomach dropping—does all the work.
Ending on a specific, small detail the character fixates on often works better than a big emotional declaration. Like, after a huge revelation, they just keep staring at the tear in the other character's jacket, the one they now realize how it got there.